What open-source projects do you contribute to or develop for?

I was compiling a list of open-source repositories, in order to learn from their development. I am evaluating contributing to at least one of them, in order to gain more experience.

The list of open-source projects need not be projects used for work, e.g. AFNetworking, but important academic developments too.

The list that I have right now is:

01.org - the list of Intel's open-source repositories

https://github.com/Itseez/opencv - opencv on github

https://github.com/pandegroup/openmm - molecular dynamics simulation, on github

https://github.com/AFNetworking/AFNetworking - A networking project for iOS devices, on github

http://developer.amd.com/tools-and-sdks/open-source/ - open-source tools from AMD.

https://developer.apple.com/opensource/

I have not had the chance to read through most of them. Because the list already contains a lot of materials, I would get the chance to learn only about a small fraction of them. Nevertheless, the list is far from complete.